But common low cost op-amps found in most consumer audio equipment just don’t have the grunt force and the finesse to actually let the headphones do what they do, which is get out of the way and deliver great music. Well, this was a first for me, building an headphone amp, so I decided I’d prototype an amp and see how it goes. The goal was as cheap as possible, with the minimum of hassle with the focus on the circuit and sound, not a final product so to speak. Here’s how it went. Vacuum tube circuit, have a simplicity unbeatable from any transistor amplifier, however, there are not many projects that use a single active component ( minimalism philosophy ).
This is a single ended transformer coupled amplifier that employs a single pentode tube D3A as active device. There are other excellent candidates : E55L, EC8020, 3A167M, 437A but all are more expensive. Normally a pentode in triode strapped reveal unlinear curve. Not so with D3A. This tube in triode mode give excellent data: amplification factor=70, trasconductance=46mA/V, plate resistance=1.5k; plate dissipation 4,5W and very linear anode curve. The perfect choice for my cheap transformer load, compact, easy-to-building, SE headphone amp.
Take a look at schema: I chose the fixed bias for D3A. Put a DC millivoltmeter across a 1 ohm resistor ( TP ) and turn VR bias trimmer until you read 16 mV ( 16 mA in the tube ) with 170 V in the anode. I noted that this is the operating point for the “right sound” with astounding results in terms of transparency and musicality ( low THD at measurement ). If you want, you can easily use the VR pot for little change and find your optimal “sweet spot” . In this application I chose Lundahl LL1689/18mA as output transformer in ALT/R ( 18:1 ) configuration ( http://www.lundahl.se/wp-content/uploads/datasheets/1689.pdf ). A primary inductance of 90 H ensure good low frequency response. Use alternative configuration ( 18:4, 18:2 ) for best match your headphone impedance. Put a GS ( grid stopper ) carbon composite 470-1K ohms resistor on tube grid pin to prevent nasty hf-influences. In power supply I suggest to use tube rectifier like 5Y3GT followed by LCLC type regulator. For the lowest hum and best L/R separation use a Mosfet capacitance multiplier stage. The 0,5 uF capacitors in the schema must be the highest possible quality ( polystyrene or polypropylene type ). About the power transformer the secondary windings must have: 240+240 V at 100mA for HT, 6,3 V at 1 A for 6C45Pi filament, 5 V at 2 A for 5Y3GT and 5 V at 100mA for negative bias.
This really was a big surprise to me! The amplifier has a most detailed, warm and very good balanced sound. Voices are very fine lifelike and excellent dynamics attacks. You can listen to this amp for days and days without your ears getting tired – music comes through fast and accurate without any buttery soupiness, but still open and airy without a hard edge. So for little money one can listen to expensive tube sound!
As mentioned above there are other excellent tube candidates : E55L, EC8020, 3A167M, 437A and so on.. but all are more expensive. I found a small triode with excellent data: amplification factor=52, trasconductance=45mA/V, plate resistance=1k; plate dissipation 7,8W the 6C45Pi. In this case the audio output transformer must have a 5Kohms primary impedance and a secondary that well match the impedance of your headphone. A good choice can be Sowter 9351 single ended output transformer ( http://www.sowter.co.uk ): it has a 5K ohms primary and 4 separate secondary windings configurable for 40-150-300-600 ohms. Adjust the bias trimmer for 35 mA in the tube with 170 V in the anode. I noted that this is the best operating point for 6C45Pi. If you want, you can easily use the VR pot for little change and find your optimal “sweet spot” . 6C45Pi has 2 grid pin connection ( 2, 8 ). Use a GS ( grid stopper ) carbon composite 470-1K ohms resistor on each pin to prevent nasty hf-influences. The power supply may be the same of D3A version or adopt a more cheaper type as tube rectifier 5Y3GT followed by CLC type configuration regulator. First capacitor MUST BE paper in oil type 4 uF/ 400-600V.